Part 2: Intracranial pressure (ICP) As mentioned in the previous tutorial, intracranial pressure is important as it affects cerebral perfusion pressure and cerebral blood flow. Normal ICP is between 5 and 13mmHg. Because it is very dependant on posture, the external auditory meatus is usually used as the zero point. Some facts and figures:- • Constituents within the skull include the brain (80%/1400ml), blood (10%/150ml) and cerebrospinal fluid (CSF 10%/150ml) • The skull is a rigid box so if one of the three components increases in volume, then there must be compensation by a decrease in the volume of one or more of the remaining components otherwise the ICP will increase (Monro-Kellie hypothesis). The term compliance is often used to describe this relationship, but it is more accurately elastance (change in pressure for unit change in volume) • Compensatory mechanisms include movement of CSF into the spinal sac, increased reuptake of CSF and compression of venous sinuses. These mech